1. Field of the Invention
The present disclosure relates to wireless communications and networking. More particularly, the present disclosure pertains to an overlay using Distributed Coordination Function (DCF) protocol for providing priority access to emergency calls.
2. Description of the Related Art
With the drafting of IEEE 802.11e standard for Quality of Service (QoS) approaching its finalization phase, issues regarding the deployment of 802.11e equipment into existing network environments with non-802.11e (legacy) equipment come to the fore. In existing non-802.11e networks (legacy networks), all terminals are treated equally with respect to gaining access to wireless channels. The IEEE 802.11e draft standard allows preferential channel allocation for traffic with QoS requirements such as multimedia streams. This preferential channel allocation is achieved through new variants for medium access control (MAC), namely extended distributed coordination function (eDCF) and the hybrid coordination function (HCF). The IEEE 802.11e specification will provide QoS enhancement at the medium access control (MAC) layer that allows WLAN systems to efficiently stream audio and video data.
802.11 WLAN is based on a cellular architecture where a system is divided into cells. Each cell is called a Basic Service Set (BSS), and is controlled by a base station called an Access Point (AP). Before transmitting frames, a station, such as a terminal device including, for example, mobile phones, PDAs, etc., must first gain access to the medium, which is a radio channel that is shared by all the stations. The 802.11 standard defines two forms of medium access, (1) distributed coordination function (DCF), and (2) point coordination function (PCF). DCF is mandatory and based on the CSMA/CA (carrier sense multiple access with collision avoidance) protocol. With DCF, 802.11 stations contend for access and attempt to send frames when there is no other station transmitting. If another station is sending a frame, stations will wait until the channel is free. The DCF and the PCF coexist and operate concurrently within a BSS. DCF may be used for Best Effort traffic delivery and PCF may be used for Real Time traffic delivery.
FIG. 1 is a timing diagram of exemplary PCF (3)/DCF (5) periods. When a point coordinator (PC) is operating in a BSS, the two access methods (DCF and PCF) alternate, with a contention-free period (CFP) 2 followed by a contention period (CP) 4 and so on occur during a CFP repetition interval 25; these periods may be dynamically adjusted on the basis of the amount of polled terminals. Each terminal that has indicated the willingness to be polled, is polled once per CFP2. A network allocation vector (NAV) 8 extends the message duration and alerts others in the medium to back off on attempting to gain access to the medium for the duration of the transmission. A beacon frame (13) 7 is transmitted before each PCF (3). A delay 11 is shown. Delay 11 is due to the medium being busy 13. After a delay 11, the subsequent CFP 2a is shortened 15.
An important aspect of the DCF 5 is a random back-off timer that a station uses when it detects a busy medium 13. If the channel is in use, the station must wait a random period of time before attempting to access the medium again. This process ensures that multiple stations wanting to send data do not transmit at the same time. The random delay causes stations to wait different periods of time and avoids all of them sensing the medium at exactly the same time, finding the channel idle, transmitting, and colliding with each other. The back-off timer significantly reduces the number of collisions and corresponding retransmissions, especially when the number of active users increases.
With radio-based LANs, a transmitting station cannot listen for collisions while sending data, mainly because the station cannot have its receiver on while transmitting the frame. As a result, the receiving station needs to send an acknowledgement (ACK) if it detects no errors in the received frame. If the sending station does not receive an ACK after a specified period of time, the sending station will assume that there was a collision (or RF interference) and retransmit the frame.
In summary, in a DCF access method, which is based on ‘listen before talk’ technology, a wireless station waits for a quiet period on the network before transmitting data and detecting any collisions. On the other hand, PCF access method goes a step further as it supports time sensitive traffic and it splits the time into contention-free and contention periods and transmits data during the former. Although these two modes offer coordination and time sensitivity, neither distinguishes between different types of traffic.
FIG. 2 shows an 802.11 protocol architectural structure for a typical wired network 20 and wireless network 202 including various types or sources of traffic 202a-202k. The 802.11 standard specifies a common medium access control (MAC) Layer, which provides a variety of functions that support the operation of 802.11-based wireless LANs. In general, the MAC Layer manages and maintains communications between 802.11 stations (radio network cards 202a-202k and access points 206, 208 by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium. Often viewed as the “brains” of the network, the 802.11 MAC Layer uses an 802.11 Physical (PHY) Layer, such as 802.11b or 802.11a , to perform the tasks of carrier sensing, transmitting, and receiving of 802.11 frames. The primary 802.11 MAC layer functions are scanning, authentication, association, WEP, RTS/CTS, power save mode and fragmentation. The wired network 204 comprises at least switches 210, 212, cables 214, 216, and the like.
As a condition to gain access to the medium, the 802.11 MAC Layer checks the value of its network allocation vector (NAV), which is a counter resident at each station that represents the amount of time that the previous station needs to send its frame. The NAV must be zero before a station can attempt to send a frame. Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the frame length and data rate. The station places a value representing this time in the duration field in the header of the frame. When stations receive the frame, they examine this duration field value and use it as the basis for setting their corresponding NAVs. Accordingly, the medium is reserved for the sending station by this process. However, the original 802.11 MAC protocol for wireless fidelity (Wi-Fi) does not support differentiation of different traffic types or sources, making it unsuitable for applications where certain traffic needs to be prioritized—such as voice or video over IP.
The implementation of the 802.11e standard will enhance both of the two modes of communications in the current state of 802.11 equipment. 802.11e defines a superset of features specified in the 1999 edition of IEEE 802.11. These enhancements distinguish QoS stations (QSTAs) from non-QoS STAs (STAs), and QoS access points (QAP) from non-QoS access points (AP). These features are collectively termed QoS facility.
When 802.11 e terminals are enabled in the presence of non-802.11 e (legacy) terminals, the QoS for non-802.11e (legacy) terminals will, in general, degrade. This is because the non-802.11e terminals are only able to send traffic in the best effort category which defers to higher priority access categories. This holds true even if the traffic from a non-802.11 e terminal is of high priority or has QoS requirements. Consequently, the QoS of Voice over IP (VoIP) calls from non-802.11e terminals tends to degrade in the presence of 802.11 e-capable equipment.
While the approach of providing only degraded best-effort service to non-802.11 e-compliant stations might be considered appropriate from an engineering standpoint, it poses many problems from business perspectives and also with respect to future governmental and regulatory requirements. Typically, technological advances are adopted gradually by enterprises, leading to a mix of equipment adhering to new and old technology standards. Consequently, the interoperability of such terminals is an important issue when deciding on the economic feasibilities of technological upgrade. Moreover, future governmental and regulatory requirements might require preferential channel access to any endpoint, such as endpoints running a VoIP connection in emergency situations. Legislatures worldwide have taken up the issues involved in migrating from traditional telephony networks to VoIP-based infrastructure, in particular with respect to emergency calls. For instance, the United States of America is working on the so-called extended 911 (E911) initiative regulating requirements for emergency calls in VoIP networks. It is to be expected that some countries will impose stringent rules requiring that emergency calls must gain preferential channel access in any type of deployment.
Unfortunately, since the existing WLANS, such as currently specified by IEEE P802.11/1999, do not support QoS transport and operate on a DCF or PCF basis, the quality degradation for non-802.11e endpoints when changing to 802.11e could require a complete upgrade to 802.11e for all stations using VoIP, potentially delaying the adoption of 802.11e in many enterprises. Further, 802.11e significantly increases the complexity of the original 802.11 MAC architecture. Most of the changes in the MAC architecture are logical consequences of introducing HCF with two new channel access functions: EDCA and HCCA. Upgrading from the original 802.11 MAC to 802.11e MAC requires extensive changes to existing functional blocks as well as adding new ones.
Further, implementation of 802.11e requires significant increases in memory, particularly RAM. The amount of additional RAM is a function of the increase in the number of transmission queues. In non-802.11e equipment, there are three queues: broadcast, multicast, and unicast. In 802.11e equipment, there are at least five: broadcast & multicast, and four access categories (AC). If hybrid coordination function (HCF) controlled channel access (HCCA) is also implemented, the number of additional queues for traffic streams varies between 1 to 8 for a QoS enhanced station (QSTA), and 1 to any number for a QoS enhanced access point (QAP) limited by available memory. Obviously, these queues and the associated buffers could be optimized to reduce the amount of RAM memory required, but the increase is still significant. This also depends on the existing software architecture of the MAC and the operating systems.
In addition, real-time constraints have become a lot tighter in 802.11e-enabled equipment. This is mainly due to the MAC level acknowledgement becoming optional. This challenge can either be overcome by a faster processor or by dedicated hardware logic; the latter being the preferred solution although expensive. The chosen approach will most likely be a compromise between performance, cost, and time-to-market.
A need therefore exists for the provision of QoS to support non-802.11e terminals with the advent of the 802.11e QoS standard. Particularly, regulatory and legislative bodies in certain countries in the near future will require preferential channel access for certain type of calls that need to be afforded with preferential treatment, such as emergency calls.